The present invention relates to broadband communications using a power line as a transmission medium and, more particularly, a current transformer installed on a power line for obtaining power from the power line.
In power-line communications (PLC), utility power lines, especially the high-voltage (HV, 60 kVAC and up) and medium-voltage (MV, 4-35 kVAC) power lines, are used as a transmission medium. The MV power lines are generally used to power the primaries of distribution transformers feeding electric power to homes and businesses. It is advantageous to convey communication signals in radio frequencies (RF).
A typical scenario in PLC is shown in FIG. 1. As shown, a main power line L1 and a number of other power lines L2, L3, L4 branching off from L1 are used to carry the RF communication signals. A server 10 is used at a distribution center to receive multimedia information from service providers and to send the information to a plurality of customers downstream. The server 10 uses an RF coupler 12 and an associated distribution modem 11 to broadcast the RF communication signals on power line L1 so that customers can receive the signals using their customer premise equipment (CPE). For example, CPE 20 and CPE 30 acquire the RF signals from L1 via RF couplers 22, 32 and associated modems 21, 31, while CPE 40 acquires the RF signals from L3 via an RF coupler 42 and an associated modem 41, and so on. On the upstream direction, customers can use their CPE to send request data to the server via the same couplers and modems.
It is known that RF signals are attenuated considerably as they are transmitted along the power line. As a result, a CPE located too far from the server 10 may not be able to receive usable RF signals. For example, while CPE 20 may be able to receive good signals from the server 10, CPEs 30, 40 and 50 may not. Thus, it is necessary to provide a plurality of repeaters 72, 74, etc. along the power lines to make it possible for CPE 30, 40 and 50 to receive the communication signals.
It should be noted that although a connection is shown from, for instance, server 10 to distribution modem 11, this connection may be via a wireless radio frequency link, e.g., according to IEEE specification 802.11x (where x=a, b, c, . . . , etc) or via a fiber optic link, etc. Such connections and methods can also be used from each of the CPEs 20, 30, 40, 50, etc. and their corresponding modems 21, 31, 41, 51, etc.
Similarly the connection from distribution modem 11 and RF coupler 12 and from each modem 21, 31, 41, 51, etc. to corresponding RF couplers 22, 32, 42, 52, etc. can be electrical (voltaic), optical or wireless.
In general, it is desirable that any server or CPE not have any physical connection (voltaic or optical fiber) to its corresponding modem if the corresponding modem is voltaically connected to its corresponding RF coupler. This general design goal is to eliminate any possible failure mode where MV voltages can be brought in contact with CPEs or servers.
When a repeater receives communication signals conveyed from the upstream direction via a power line, it is designed to repeat the communication signals so that the CPE in the downstream can receive useful RF signals. These repeated signals will also travel upstream along the same power line. When there are many repeaters along the same power line repeating the same communication signals, there will be significant interference among the repeated signals because of the delay in each repeater and the overlap of signals. In general, a repeater is needed at a location when the communication signals have been attenuated significantly but are still useful.
In power-line communications (PLC) as mentioned above, a current transformer operating at the utility frequency (50 or 60 Hz) can be used to obtain an induced current for powering the RF couplers 12, 22, 32, 42, 52 and the repeaters 72, 74, 76, for example. The same current transformer can also be used to power power-line current measurement equipment. If the current transformer is installed on an already operating power line, the current transformer must use a split core to develop power by magnetic induction.
The split core in a current transformer comprises at least two magnetically permeable parts, each shaped like a half donut, for example. When the current transformer is installed on an active, current-carrying power line, the split core parts must be closed around the power line to form a substantially closed-loop transformer core. Before the split core parts are completely closed, there will be a gap between the core parts. Because the current in the active power line creates a spatially nonlinear magnetic field near the surface of the conductor carrying the current, the magnetically permeable material of the split core parts will experience forces exerted by the nonlinear magnetic field. These forces are concentrated in the core gap in the open split core parts, and their magnitude is inversely proportional to the fourth power of the distance of the core gap. As the split core parts are closed onto each other to form a substantially closed-loop, the forces increase very rapidly and they may cause the split core to slam together. The slamming action can cause damage to the current transformer.
When the current transformer is removed from the active power line, it is necessary to create a gap in the split core parts. The same nonlinear magnetic field will exert an attractive force on the gap, preventing the gap from being widened. As a result, the counter-force required to open the split core in order to remove the current transformer from the active line may be larger than practical. Furthermore, once a gap is formed and it exceeds a certain distance, the reduction in the attractive force is significant and sudden, resulting in possible damage to the core if the split core parts are separated too rapidly.
Thus, it is advantageous and desirable to provide a method and device for reducing or eliminating the magnetic forces developed on the split core parts prior to the split core parts being closed to form a closed-loop in order to avoid damage to the split core parts. The same method and device can be used to reduce the counter-force necessary for opening the split core parts for removal.
It is a primary objective of the invention to reduce or eliminate the magnetic forces exerted on the split core parts of a current transformer when the current transformer is installed on an active, current-carrying power line and when the split core parts are opened for the removal of the current transformer from the power line. This objective can be achieved by shorting the multiple-turn winding on the split core parts during the installation and removal of the current transformer.
Thus, according to a first aspect of the present invention, there is provided a method of reducing magnetic forces exerted on a current transformer positioned about a current-carrying conductor, wherein the current transformer comprises a magnetically permeable core having at least two split core parts separable by a gap, and wherein the gap can be closed so as to allow the split core parts to form a substantially closed-loop around the current carrying conductor in a closed configuration, and the gap can be widened so as to allow the current transformer to be removed from the current-carrying conductor, and wherein the current transformer further comprises a winding having a plurality of turns of an electrical conductor wound around the magnetically permeable core. The method comprises the steps of
shorting the winding prior to closing the gap between the split core parts for achieving the closed configuration, and
shorting the winding prior to separating the split core parts from each other if the split core parts are in the closed configuration.
According to a second aspect of the present invention, there is provided a device for reducing magnetic forces exerted on a current transformer positioned about a current-carrying conductor, wherein the current transformer comprises a magnetically permeable core having at least two split core parts separated by a gap, and wherein the gap can be closed so as to allow the split core parts to form a substantially closed-loop around the current-carrying conductor in a closed configuration, and the gap can be widened so as to allow the current transformer to be removed from the current-carrying conductor, and wherein the current transformer further comprises a winding having a plurality of turns of an electrical conductor wound around the magnetically permeable core. The device comprises a mechanism capable of
a shorting device in operative engagement with the winding so as to be able to short the winding; and
a mechanism, positioned relative to the split core parts so as to be able to close the gap between the split core parts or to separate the split core parts from each other.
According to the third aspect of the present invention, there is provided a current transformer to be positioned about a current-carrying conductor. The current transformer comprises:
a magnetically permeable core having at least two split core parts separable by a gap, wherein the gap can be closed so as to allow the split core parts to form a substantially closed-loop around the current-carrying conductor in a closed configuration, and the gap can be widened for separating the split core parts from each other so as to allow the current transformer to be removed from around the current-carrying conductor;
a winding having a plurality of turns of an electrical conductor wound around the magnetically permeable core; and
a shorting device positioned relative to the winding so as to be able to:
short the winding prior to closing the gap, and to be able to
short the winding prior to separating the split core parts if the split core parts are in the closed configuration.